1.1 Field of the Invention
The invention relates to coke cutting methods in delayed cokers. More particularly, the invention relates to a method for determining whether and where coke cutting is required using vibration signature analysis.
1.2 Description of Related Art
Delayed coking is a process for the thermal conversion of heavy oils such as petroleum residua (also referred to as “resid”) to produce liquid and vapor hydrocarbon products and coke. Delayed coking of resids from heavy and heavy sour (high sulfur) crude oils is carried out by converting part of the resids to more valuable liquid and gaseous hydrocarbon products. The resulting coke has value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), etc.
In the delayed coking process, the feed is rapidly heated at about 500° C. (932° F.) in a fired heater or tubular furnace. The heated feed is conducted to a coking vessel (also called a “drum”) that is maintained at conditions under which coking occurs, generally at temperatures above about 400° C. (752° F.) and super-atmospheric pressures. Coke drums are generally large, upright, cylindrical, metal vessels, typically ninety to one-hundred feet in height, and twenty to thirty feet in diameter. Coke drums have a top portion fitted with a top head and a bottom portion fitted with a bottom head. Coke drums are usually present in pairs so that they can be operated alternately. Coke accumulates in a vessel until it is filled, at which time the heated feed is switched to the alternate empty coke drum. While one coke drum is being filled with heated residual oil, the other vessel is being cooled and purged of coke.
The heated feed forms volatile species including hydrocarbons that are removed from the drum overhead and conducted away from the process to, e.g., a fractionator. The process also results in the accumulation of coke in the drum. When the first coker drum is full of coke, the heated feed is switched to a second drum. Hydrocarbon vapors are purged from the coke drum with steam. The drum is then quenched with water to lower the temperature to a range of about 93° C. to about 148° C. (about 200° F. to about 300° F.), after which the water is drained. When the cooling step is complete, the drum is opened and the coke is removed by drilling and/or cutting. The coke removal step is frequently referred to as “decoking”.
Current coke cutting practices for delayed coker drums require the drilling of a pilot hole to create a passage to the bottom outlet of the drum, followed by stepwise cutting of the coke bed from the top to the bottom of the drum. A cutting/boring tool is located on a drill stem that conducts water to nozzles on the tool which create water jets. A hole is typically bored in the coke by water jet nozzles oriented vertically on the head of the cutting/boring tool. Similarly, nozzles oriented horizontally on the head of the cutting/boring tool cut the coke from the drum. The coke is typically cut from the drum using a low speed (with rpm around 15-20), high impact water jet. The coke removal step adds considerably to the throughput time of the process. Drilling and removing coke from the drum takes approximately 1 to 6 hours. The coker drum is not available to coke additional feed until the coke removal step is completed, which negatively impacts the yield of hydrocarbon vapor from the process. Coke cutting is typically a manually controlled process with the individual running the cutting system relying on visual appearance of the drum discharge and, to a lesser extent, on audible clues from contact of the cutting water with the drum wall.
Recently, various methods have been developed by ExxonMobil Research and Engineering Company (EMRE) for generating coke in a substantially free-flowing form, such as a free flowing shot coke, which is more easily removed from the drum. (See, e.g., US 2003/0102250; US 2004/0256292; US 2005/0284798; US 2006/0006101; US 2006/0060506; and US 2006/0196811.) Substantially free-flowing coke is particularly suited to removal by a decoking process also developed by EMRE referred to as “slurry decoking.” (See, e.g., U.S. 2005/0269247.)
In slurry decoking, the coke is formed into a slurry in the coker vessel prior to its removal from the vessel. The slurry is formed when quench water floods the hot coker drum for cooling purposes. In conventional processes, the water would be drained from the coker drum before coke cutting and subsequent coke removal. But in “slurry decoking”, contrary to conventional practices, the quench water is allowed to remain in the coker drum after cooling and to form a slurry with the coke. By skipping the traditional drain step, and discharging a coke water fluid, significant savings in cycle time can be achieved, which may translate to higher potential unit throughput.
With the advance of improved methods for generating free-flowing coke, and techniques for processing the same such as slurry decoking, the amount of coke required to be cut and the time required for cutting/polishing a drum can be markedly reduced because the bulk of the loose coke formed will be discharged from the drum without having to be cut. Ideally, the cutting step is completely eliminated. However, current expectations and observations are that some cutting is still required to adequately clean the drum for the next cycle in at least some instances. Nonetheless, cutting time is reduced because less coke remains in the drum to be removed.
To maximize these improvements in cycle time, there is a need for a method that identifies whether cutting is or is not required during a given cycle. Furthermore, if cutting is required, there is a need for a method that identifies the specific areas on the drum that require cutting and that targets those areas. Finally, it would be desirable to have a method of controlling coke cutting that eliminates the need for operators to rely on their subjective, and inherently uncertain and variable, assessment of the process based visual appearance and audio clues.